The present invention relates to an air-fuel ratio control method for internal combustion engines, and more particularly to a method of controlling the air-fuel ratio of a mixture of fuel supplied to an internal combustion engine when the engine has shifted from a high-load operation to a low-load operation.
Conventionally, an air-fuel ratio control method for internal combustion engines is known, e.g., from Japanese Provisional Patent Publication (Kokai) No. 58-160528, which controls the air-fuel ratio of a mixture supplied to the engine so as to improve the fuel consumption and emission characteristics, etc., of the engine, by sensing the concentration of an ingredient of exhaust gases emitted from the engine by means of an exhaust-gas concentration sensor provided in the exhaust system of the engine, effecting feedback control in response to the sensed ingredient concentration to bring the air-fuel ratio to a desired set value, and interrupting the feedback control during engine operation under particular operating conditions and effecting open-loop control to bring the air-fuel ratio to set values different from the above desired set value but respectively suitable for the particular operating conditions.
According to the conventional control method, as the engine shifts from the high-load operation to the low-load operation, three kinds of control are sequentially effected, as shown in FIG. 8. That is, open-loop control is first effected to decrease the air-fuel ratio to a value smaller than the above desired set value provided for feedback control, for enriching the mixture in a high-load operating region, i.e., a wide-open-throttle region, feedback control is then effected in a medium-load operating region, i.e., a feedback control region, and open-loop control is effected to increase the air-fuel ratio to a value larger than the above desired set value, for leaning the mixture in a low-load operating region, i.e., a mixture leaning region, in the mentioned order. In the mixture leaning region, the supply of fuel to the engine is interrupted (fuel cut) when a predetermined operating condition of the engine is satisfied.
However, the method has a disadvantage if the above sequential control is effected when the engine shifts from the wide-open-throttle region to the mixture leaning region in a brief time after a long-term staying in the wide-open-throttle region. That is, while mixture-enriching control has been effected for a long time period in the wide-open-throttle region, a large amount of fuel adheres to the inner wall of the intake pipe, throttle valve, etc. The fuel still adhering to the inner wall, etc. at the departure from the wide-open-throttle region is little drawn into combustion chambers while the engine is passing the feedback control region, because the engine stays in the feedback control region for a short period of time. Thus, most of the fuel remains within the intake pipe even after the engine has then shifted to the mixture leaning region. However, once the engine has shifted to the mixture leaning region, the adhering fuel is drawn in large quantities into the combustion chambers due to a decrease in absolute pressure within the intake pipe, whereas, in the mixture leaning region, the throttle valve is fully closed or slightly opened and hence the intake air introduced into the intake pipe is small in amount. Consequently, a very overrich mixture is supplied into the combustion chambers. As a result, part of the mixture will not be burnt within the combustion chambers, and the unburnt fuel is emitted from the combustion chambers into the engine exhaust system, thereby causing so-called after-fire within the exhaust system. Particularly, if the engine is provided with a three-way catalyst as an exhaust-gas purifying device in the exhaust system, such after-fire causes excessive rise of the temperature of the three-way catalyst, thereby deteriorating the performance of same and hence shortening the service life thereof. On the other hand, even if the entire mixture is burnt within the combustion chambers, the mixture which is overrich as stated above causes increase of CO and/or HC ingredients in the exhaust gases and hence degrade the emission characteristics.
Particularly, in an engine of a type having fuel injection valves arranged both upstream and downstream of the throttle valve, the intake pipe has a long span between the injecting location of the upstream valve and the combustion chambers so that the amount of fuel adhering to the intake pipe etc. becomes considerably large, thus making the above problem more serious.